1. Field of the Invention
The invention relates to coherent optical receivers and methods of operating coherent optical receivers.
2. Discussion of the Related Art
This section introduces aspects that may be helpful to facilitating a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In optical communication systems, a variety of schemes have been considered for implementing coherent detection. While homodyne detection is often desirable, there is a significant difficulty with implementing homodyne detection in optical communications systems at high data rates. Homodyne detection involves phase locking to the received carrier, and phase locking is often complex to achieve for an optical carrier. Partially due to this deficiency of optical homodyne detection, heterodyne and intradyne detection have also been considered for coherent optical receivers.
In intradyne detection, the receiver uses a local oscillator to down mix the data-carrying carrier. Nevertheless, the local oscillator of the receiver is not tightly phase or frequency locked to the data-carrying optical carrier. Instead, the local oscillator of the receiver has a frequency that loosely configured to be closer to the frequency of the data-carrying carrier than the bandwidth of the data-carrying carrier. Thus, an intradyne optical receiver would not typically have an optical phase locked loop. Due to the absence of optical phase locked loops, intradyne optical receivers may offer advantages in coherent optical detection.
Coherent detection may be used to recover data transmitted via a phase-shift-keying (PSK) modulation scheme. Nevertheless, the performance of a receiver of PSK modulated data is typically sensitive to frequency offsets and carrier linewidths. For that reason, efficiently implementing an intradyne optical receiver for PSK modulated optical carriers has posed some challenges.